Bipolar transistors are known in the art. Metal Insulator Semiconductor (MIS) transistors are known in the art. A prior art Lateral MIS transistor is represented in FIG. 1. A very thin layer of insulator 50 such as SiO.sub.2, between the metal emitter 52 and metal collector 54 and semiconductor base 56 prevents current flow between the base 56 and emitter 52 or collector 54. However, when V.sub.be is large enough, the emitter 52 injects electrons through the insulator 50 to the base 56. The emitter 52 and collector 54 are further isolated from each other, laterally, by thick oxide 58, which separates them by distance d.sub.B. The base contact 60 is separated from the collector 54 by thick field oxide 62, at a distance d. The base contact 60 is through a highly doped region 64 (P.sup.+ or N.sup.+) which insures ohmic contact to the base 56. Since the collector 54 is reverse biased with respect to the base 56, electrons are drawn from the body to the surface of the base 56 at the thin oxide 50 under the collector 54 to form a depletion region there (not shown). This depletion region, which is set in part by base dopant level, controls base 56 to collector 54 current. With optimum thin oxide thickness at the collector 54 and optimum base dopant level, the collector can saturate at low levels, exhibiting collector current characteristics similar to traditional bipolar transistors.
Additionally, controlling the depletion regions controls collector current saturation. Increasing the depletion region increases the Electric field (E-field) across the collector thin oxide, increasing collector current.
Unfortunately, there are several phenomena that limit the current gain and, as a consequence, any performance improvement realized from lateral MIS transistors. Primarily shunt current, mainly high base-to-emitter current leakage, limits lateral MIS transistor current gain. Shunt current occurs because carriers from the emitter 52 tend to follow downward into the base 56, rather than horizontally into the collector 54. Some carriers recombine in the lower regions of the base 56 rather than exit into the collector 54. Thus, these recombining carriers, which are part of the base current, exhibit the characteristics of a resistive (linear) current, i.e. exhibit the characteristic of a shunt resistor between the base 56 and the emitter 52. Consequently, the shunt current, which is not amplified, reduces current gain.
Heterojunction bipolar transistors (HBTs) are known in the art. A heterojunction has at least two layers of at least two dissimilar semiconductor materials. These two materials have different energy at the conduction and valence band edges (E.sub.c and E.sub.v respectively) and different electron affinities. Thus, the heterojunction layer interface creates band gap spikes .DELTA.E.sub.c and .DELTA.E.sub.v. .DELTA.E.sub.c and .DELTA.E.sub.v reinforce current flow in one (forward biased) direction and reduce current flow in the other (reverse biased) direction. Because electrons effectively will tunnel through the conduction potential spike, .DELTA.E.sub.c, this increased forward current flow is also known as tunnel current. Thus, properly biased HBTs have an increased DC current gain over traditional homojunction bipolar transistors.